Temperature-Pressure Phase Diagrams of Proteins

Laszlo Smeller 

Semmelweis University of Medicine, Institute of Biophysics and Radiation Biology, Budapest, Hungary


It is well known that the specific, biologically active spatial
structure of proteins is stable only in a limited range of pressure
and temperature []. When the stable region is plotted on the
pressure-temperature plane, it occupies an ellipse-like area. This
elliptic phase diagram explains, why the protein can be unfolded by
subjecting it to heat, pressure or cold.
The thermodynamical description of the elliptic phase diagram can be
obtained by writing the DG (=G[denatured]-G[native]) as function of
the changes in volume, compressibility, heat capacity, thermal
expansion coefficient at the phase boundary. The shape and position of
the ellipse therefore contains information about the thermodynamic
parameters of the unfolding. It has to be noted that, in spite that
this description is based on equilibrium thermodynamics, by plotting
the isokineticity curves similar elliptic shapes can be obtained.
If the protein is subjected to moderate pressure-temperature effects,
remaining inside of the ellipse, elastic changes occur in the
structure. The secondary structure is not disrupted. Crossing the
elliptic boundary plastic conformational changes occur.
The elliptic boundary separates the p-T space into two areas, and
therefore distinguishes only two main protein conformations: native
and denatured (unfolded). However several investigations showed
evidence that the structures of protein exposed to heat, pressure and
cold denaturations are different. It was shown by infrared
spectroscopy that the heat induced unfolding is followed immediately
by an aggregation, leading to a gel stabilised by intermolecular
antiparallel beta sheet type hydrogen bonds, showing specific infrared
bands. This antiparallel beta structure cannot be found in the
pressure or cold denatured proteins. This suggests that the unfolded
protein can build at least two different type of gel outside the
elliptic boundary.
Recent FTIR investigation carried out on myoglobin to compare the
secondary structures adopted by the protein following different ways
of denaturation, underlined the role of intermediate structures. The
aggregation process was also studied as a function of pressure and
temperature. It was shown that pressure-unfolded proteins have an
increased tendency for the aggregation. The aggregated protein could
however partially be disaggregated by applying moderated pressures

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Presentation: oral at High Pressure School 1999 (3rd), by Laszlo Smeller
See On-line Journal of High Pressure School 1999 (3rd)

Submitted: 2003-02-16 17:33
Revised:   2009-06-08 12:55
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